1. Field of the Invention
The present invention relates to a method and an apparatus for identifying plastics of unknown materials.
2. Description of the Related Art
To identify materials of unknown objects in the art, JP 7-111397(B2) and JP 8-300354(A) publications describe methods of applying infrared rays on objects to identify them on the basis of differences, for example, in absorption spectrum, and amount of transmitted and reflected infrared rays.
To classify waste plastics into their types, JP 7-124053 application describes a method and apparatus for classifying plastics, which heats plastic fragments with microwaves and using a temperature difference between plastics. This classifying apparatus comprises: a crushing device for crushing waste plastics into a given size; a heating furnace for preheating crushed plastic fragments; a microwave oven for heating the preheated plastic fragments with microwaves; a radiation thermometer for measuring temperatures of the plastic fragments; and a classifying section for classifying the plastics on the basis of the results of measured temperatures. In the microwave heating, heat efficiency depends on permittivity. Therefore, the apparatus measures temperatures of plastic fragments after microwave heating and uses airflow to blow away only such plastic fragments for classification that are heated up to a temperature within a given range. In this apparatus, the heating furnace for preheating is used to enlarge differences in temperatures of plastics.
JP 8-113391 application describes a method and an apparatus for classifying plastics, which heats plastics with infrared rays and uses the temperature difference between plastics. This method and apparatus demonstrates examples identified through the use of temperature differences resulting from amounts of infrared rays absorbed by polyethylene terephthalate (hereinafter referred to as xe2x80x9cPETxe2x80x9d) and polyvinyl chloride (hereinafter referred to as xe2x80x9cPVCxe2x80x9d).
JP8-122868 application describes a method and an apparatus for classifying plastics, which frictionally electrifies plastic fragments in an electrifying container and uses differences in polarity and charged amount between the plastic fragments. When plastics of different types are frictionally contacted with each other, their electrostatic properties and charged amounts tend to differ in accordance with materials. In this apparatus, the plastic fragments are frictionally contacted with a frictional medium in the electrifying container. Then, the charged plastic fragments are supplied through electrodes that have a high potential therebetween to classify them for collection into one that adheres on one electrode, one that does not adhere on electrodes, and one that is attracted to the other electrode. A classifying apparatus using this method includes an electrostatic plastic separation device available from Hitachi Zosen Inc.
The conventional method, which uses differences in absorption, transmission and reflection properties for infrared rays, has a disadvantage because it fundamentally uses a ray of light and can not identify materials of colored objects. In particular, in wastes, objects are hardly identified because they are in various forms and mixed.
The method, which uses temperature differences resulting from heating with microwaves, has an extremely high possibility to generate discharges (sparks) when a metal is mixed in objects to be heated and a risk to fire plastics. In addition, JP 7-124053 preheats plastics at 80xc2x0 C. for increasing an identification rate but temperatures of the plastics finally elevate near to or over 100xc2x0 C. occasionally. In such a case, there is a risk to melt or decompose the plastics partly, which gives a large disadvantage for safety. Furthermore, even if identifying of plastics succeeds, it is difficult to blow plastic fragments in order to efficiently identify because they do not have a constant weight. Accordingly, this method is not possible to identify the plastic products that are not crashed. Further, the microwave oven has a large size and requires microwave shields in consideration of influences on human beings. As obvious from the forgoing, the apparatus, which uses microwave heating, is hardly applicable to a waste processing system that requires a high identification rate, a low cost and high safety.
The method and apparatus for identifying plastics, which heats plastics with infrared rays and uses the temperature difference between plastics, cannot identify PVC from other plastics. In consideration of a period of time from heating to measurement, a measured error and stability of the heating device, about 5 degrees of Centigrade is an enough temperature difference to identify temperatures by this method. Polymethyl methacrylate (Methacrylic resin; hereinafter referred to as xe2x80x9cPMMAxe2x80x9d) and polystyrene (hereinafter referred to as xe2x80x9cPSxe2x80x9d), however, cannot be identified from PVC. The apparatus described in JP 8-113391 demonstrates examples identifying PVC bottles from PET bottles. Its identification rate is extremely lowered when plastics of other types than the above and plastics of other forms than bottles are mixed.
The method and apparatus for classifying plastics, which frictionally electrifies plastic fragments in the electrifying container and uses differences in polarity and charged amount between the plastic fragments, is difficult to perform identification efficiently. For example, in identifying a mixture of PVC fragments and polyethylene (hereinafter referred to as xe2x80x9cPExe2x80x9d) fragments, when PVC fragments contact PE fragments in the electrifying container, PE fragments tend to be charged positively and PVC fragments, negatively. When PE fragments contact with each other or when PVC fragments contact with each other, electrostatic polarity and charged amounts greatly vary in accordance with frictional situations. Even if PE fragments are charged positively and PVC fragments, negatively, when weights and sizes of plastic fragments are different, electrostatic force and gravity imparted on plastic fragments may differ. Therefore, the method of identifying, depending on electrostatic force as described in JP 8-122868, is difficult to perform identification efficiently. Thus, a crushing step is essentially required to equate sizes of plastics. It is extremely difficult, however, to crush all waste plastics in various forms into a given particle diameter and greatly increases the cost. Furthermore, an increased number of plastic fragment types make it more difficult to control charged amounts and impossible to identify general waste plastics.
On the other hand, in appropriately processing and recycling wastes, there is a high possibility to generate toxic substances on burning and heating the wastes if PVC is mixed therein, and a technique is needed to identify and remove PVC from the wastes. It is very difficult, however, for the conventional method to identify and remove only PVC. If the wastes with PVC mixed therein are burned easily, toxic substances, represented by dioxin, are possibly generated due to chlorine derived from PVC. A proposed method can make toxic substances harmless just when they are generated, but it requires an extremely expensive chemical facility and is not practical.
In reusing collected plastics as a refuse derived fuel (hereinafter referred to as xe2x80x9cRDFxe2x80x9d) or a raw material for a blast furnace, it is very important to increase qualities of the collected plastics. For example, an RDF is required to have a high calorific value and a constant amount of heat. If a high exothermic plastic and a low exothermic plastic are mixed, such an RDF cannot be obtained. In particular, for a power generation system that uses an RDF, a high-quality RDF is requested. Currently, the high-quality RDF is not developed progressively, and there is a disadvantage in that an efficiency of power generation is extremely low. In the raw material for the blast furnace, it is required to mix high exothermic plastics and low exothermic plastics in a constant mixing amount in order to run the blast furnace in stably. Furthermore, if PVC is mixed in an RDF or in the raw material for the blast furnace, there is a high possibility to generate a toxic gas and a corrosive gas such as chlorine due to burning. These gases impart a great deal of influence on human beings and the blast furnace system, and adequate countermeasures are required.
If the wastes are crashed into small fragments, desired materials are hardly identified and collected efficiently from the crushed fragments. In consideration of crashing processes, dust countermeasures, noise countermeasures, treatments of collected materials and costs for transportation and so forth, it is preferred to construct a system that can identify the collected products as such on material basis, collect and supply them to the following process step.
Accordingly, a subject of the present invention is to provide a method and an apparatus for identifying waste plastics efficiently. More particularly, it is to provide a method and an apparatus for identifying waste plastics, which contain coloring agents and are in various forms, into three groups consisting of high exothermic plastics, low exothermic plastics and PVC, and for collecting them without crushing while maintaining their product forms.
Accordingly, the present invention is embodied as follows:
A first aspect of the present invention is a method of identifying plastics comprising: heating and electrifying a mixture of at least two plastics; and measuring heating properties and electrostatic properties thereof to identify the plastics.
A second aspect of the present invention is a method of identifying plastics comprising: heating, followed by electrifying, a mixture of at least two plastics; and measuring heating properties and electrostatic properties thereof to identify the plastics.
A third aspect of the present invention is a method of identifying plastics comprising: electrifying, followed by heating, a mixture of at least two plastics; and measuring electrostatic properties and heating properties thereof to identify the plastics.
A fourth aspect of the present invention is a method of identifying plastics comprising: simultaneously heating and electrifying a mixture of at least two plastics; and measuring heating properties and electrostatic properties thereof to identify the plastics.
Suitable conditions are described hereunder to carry out any one of the above-mentioned four aspects of the present invention.
The mixture includes at least one low exothermic plastic, at least one high exothermic plastic, and polyvinyl chloride.
The mixture is heated to identify the high exothermic plastic on the basis of a difference in heating property, followed by electrifying a mixture of the low exothermic plastic and the polyvinyl chloride to identify the polyvinyl chloride on the basis of a difference in electrostatic property.
The mixture is electrified to identify the low exothermic plastic on the basis of a difference in electrostatic property, followed by heating a mixture of the high exothermic plastic and the polyvinyl chloride to identify the polyvinyl chloride on the basis of a difference in heating property.
The heating comprises radiation of far infrared rays.
The electrifying comprises any one of corona discharge, frictional electrification, and a combination of corona discharge and frictional electrification.
The low exothermic plastic has, as essential components, polystyrene, polyethylene terephthalate, acrylonitrile-butadiene-styrene copolymer and methacrylic resin.
The high exothermic plastic has, as essential components, polyethylene and polypropylene.
The mixture comprises plastic products or wastes of plastic products.
The mixture is not crashed.
A fifth aspect of the present invention is an apparatus for identifying plastics comprising: a heating device for heating the plastics; an electrifying device for electrifying the plastics; a temperature-measuring device for measuring temperatures of the plastics; an electrification-measuring device for measuring charged states of the plastics; a display for displaying measured results of the temperatures; and a display for displaying measured results of the charged states, wherein the apparatus is operable in accordance with the method of identifying plastics as recited in any one of the above-mentioned four aspects of the present invention.
Suitable conditions are described hereunder to carry out the above-mentioned fifth aspect of the present invention.
The heating device comprises a device that uses far infrared rays, and a device that comprises a far infrared heater and a concave reflector, the reflector collecting far infrared rays to apply them on an object to be heated.
The electrifying device comprises a device that is operable in a way according to any one of corona discharge, frictional electrification, and a combination of corona discharge and frictional electrification, a device that comprises a plurality of stylus-like protrusions; corona discharge electrodes configured with fixed portions of the protrusions; and a power supply for applying a DC or AC voltage on the corona discharge electrodes, and a device that comprises a frictional medium; a fixing section for fixing the frictional medium; and a containing section for containing the frictional medium.
The frictional medium is one selected from the group consisting of a roller, a blush and a rod.
The electrifying device comprises a roller-like frictional medium and an auxiliary roller opposite thereto, the electrifying device interposing the plastics between the frictional medium and the auxiliary roller to frictionally electrifying the plastics.
The frictional medium has a portion composed of polyvinyl chloride for coming into contact with the plastics.
The polyvinyl chloride comprises soft polyvinyl chloride alone or a mixture of soft polyvinyl chloride and a plasticizer of 25-50% by weight of the mixture.
The temperature-measuring device comprises a device for outputting the measured results of temperatures to the display as temperature image data.
The electrification-measuring device comprises a device for measuring surface charges, surface potentials, or surface charges together with surface potentials on the plastics and outputting them to the display as charged data.
The display comprises a device for image-projecting the temperature image data input as such on the object measured by the temperature-measuring device, a device for image-projecting the charged data input as such on the object measured by the electrification-measuring device, or a marking means for adding identified marks on the plastics, the mark being classified on the basis of measured data input.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof.